Structural Effects of Imidazolium-Based Ionic Liquids on the Depolymerization Efficiency of Polycarbonate = 이미다졸리움 기반 이온성 액체의 구조가 폴리카보네이트 해중합 효율에 미치는 영향

다국어 초록 (Multilingual Abstract)

Plastics are essential materials widely used across industrial and daily life applications due to their excellent mechanical properties and flexibility. However, the growing volume of plastic waste has emerged as a serious environmental concern. In particular, polycarbonate (PC), a high-performance engineering plastic widely used in electronics, food containers, and automotive parts, has seen a surge in consumption and associated waste. Owing to its low recyclability, PC is typically disposed of via incineration or landfilling, during which bisphenol A (BPA)—its primary monomer and a known endocrine-disrupting chemical—may leach into the environment. Therefore, developing an effective chemical recycling process that enables monomer recovery from PC is urgently needed.
This dissertation aims to establish a PC ethanolysis process utilizing ionic liquids (ILs) as both catalyst and solvent. Specifically, imidazolium-based ILs were systematically designed and modified to investigate how variations in cation alkyl chain length and branching, as well as the presence and accessibility of hydroxyl groups in anions, affect depolymerization efficiency.
The results revealed that ILs with short, linear alkyl chains and anions bearing accessible hydroxyl groups significantly promoted hydrogen bonding with PC, leading to enhanced PC solubility and depolymerization performance. Furthermore, it was demonstrated that not only the presence of reactive functional groups, but also their structural accessibility for interaction with external molecules, is critical to efficient depolymerization.
In addition, purification processes were developed to improve the purity of recovered BPA, enabling its reuse in PC polymerization. A recovery process for diethyl carbonate (DEC), a value-added byproduct applicable as a lithium-ion battery electrolyte, was also established. These developments contribute to improving the economic feasibility and industrial applicability of the proposed recycling process.
Overall, this work provides a molecular-level design strategy based on the structure–activity relationship of ILs and offers new directions for the chemical recycling of polymers, ultimately contributing to the realization of a sustainable plastic circular economy.

국문 초록 (Abstract)

플라스틱은 우수한 기계적 물성과 유연성으로 인해 산업 및 일상생활 전반에 걸쳐 필수적으로 사용되고 있으나, 그에 따른 폐기물의 급증은 심각한 환경 문제로 대두되고 있다. 특히, 고기능성 엔지니어링 플라스틱인 폴리카보네이트 (PC)는 전자기기, 식품 용기, 자동차 부품 등 다양한 분야에서 사용량이 증가하고 있으며, 이에 따라 폐기물 배출량도 증가하고 있다. PC는 낮은 재활용률로 인해 주로 소각 또는 매립되며, 이 과정에서 주요 단량체이자 내분비계 교란 물질인 비스페놀 A(BPA)가 환경으로 유출될 수 있다. 이에 따라, PC로부터 단량체를 효과적으로 회수할 수 있는 화학적 재활용 공정의 개발이 필요하다.
본 학위논문에서는 이러한 문제를 해결하기 위한 전략으로 이온성 액체(ionic liquids, ILs)를 촉매 및 용매로 활용하는 PC의 에탄올 분해(ethanolysis) 공정 개발을 목표로 하였다. 특히, 이미다졸리움 기반 IL의 양이온 알킬 사슬 길이 및 가지 유무, 음이온의 하이드록실기 존재 및 접근 가능성 등 구조적 변수를 체계적으로 조절함으로써, IL의 분자 구조가 해중합 효율에 미치는 영향을 분석하였다.
그 결과, 짧고 선형적인 알킬 사슬을 가지는 양이온과 외부와의 수소결합이 가능한 하이드록실기를 지닌 음이온을 조합할 경우, 혼합물 내에서 PC와의 수소결합이 강화되어 PC 용해도 및 해중합 효율이 크게 향상됨을 확인하였다. 특히, 단순히 작용기가 존재하는 것만으로는 충분하지 않으며, 해당 작용기가 외부 분자와 상호작용할 수 있는 구조적 접근성을 지니는 것이 해중합 효율 향상의 핵심임을 입증하였다.
더불어, 회수된 BPA의 고순도화를 위한 정제 공정을 개발하여 재중합에 활용 가능한 수준(99.5%)으로 순도를 향상시켰으며, 리튬 이차전지 전해질로 활용 가능한 부가가치 물질인 diethyl carbonate (DEC)를 회수하는 공정도 확립하였다. 이는 본 해중합 공정의 경제성과 산업적 적용 가능성을 높이는 데 기여한다.
결론적으로, 본 연구는 IL의 구조–활성 상관관계에 기반한 분자 설계 전략을 제시함으로써 고분자의 화학적 재활용 기술 개발에 새로운 방향을 제시하며, 지속가능한 플라스틱 순환 체계 구축에 실질적으로 기여할 수 있을 것으로 기대된다.

목차 (Table of Contents)

• ABSTRACT i
• 국문 초록 iii
• PREFACE vi
• TABLE OF CONTENTS vii
• LIST OF TABLES xi
• LIST OF FIGURES xii
• CHAPTER 1. INTRODUCTION １
• 1.1 The urgency of plastic waste management and the need for polycarbonate recycling １
• 1.1.1 Global environmental challenges from plastic waste １
• 1.1.2 Characteristics of polycarbonate and its recycling challenges ４
• 1.2 Recycling approaches of polycarbonate ７
• 1.3 Classification of chemical recycling methods of polycarbonate １１
• 1.4 Ionic liquids as solvents and catalysts in depolymerization １５
• 1.5 The Aim of the Dissertation ２１
• CHAPTER 2. EFFECT OF ALKYL CHAIN STRUCTURE OF IMIDAZOLIUM-BASED IONIC LIQUID ON THE ETHANOLYSIS OF POLYCARBONATE ２２
• 2.1 Introduction ２２
• 2.2 Materials and Methods ２３
• 2.2.1 Materials ２３
• 2.2.2 Synthesis of 1-(sec-Butyl)-3-methylimidazolium bromide ([sec-BMIM][Br]) ２３
• 2.2.3 Synthesis of 1-(iso-Butyl)-3-methylimidazolium bromide ([isoBMIM][Br]) ２３
• 2.2.4 Synthesis of 1-(tert-Butyl)-3-methylimidazolium iodide ([tert-BMIM][I]) ２４
• 2.2.5 Halide-to-anion switch for imidazolium-based ionic liquids using AER ２４
• 2.2.6 COSMO-RS calculations ２４
• 2.2.7 Ethanolysis of PC ２５
• 2.2.8 Characterization of the reactants and products ２６
• 2.3 Results and Discussion ２７
• 2.3.1 Catalyst Screening ２７
• 2.3.2 Parameters related to PC solubility predicted by COSMO-RS ３５
• 2.3.3 Optimizing the reaction parameters of PC depolymerization ４３
• 2.3.4 Reusability of [n-BMIM][Ac] ４６
• 2.4 Conclusions ４８
• CHAPTER 3. EFFECT OF ALKYL CHAIN LENGTH OF IMIDAZOLIUM-BASED IONIC LIQUID ON PC ETHANOLYSIS ４９
• 3.1 Introduction ４９
• 3.2 Materials and Methods ５０
• 3.2.1 Materials ５０
• 3.2.2 Conductor-like screening model for real solvents (COSMO-RS) calculations ５１
• 3.2.3 Protocol for the ethanolysis of PC ５２
• 3.2.4 Synthesis of polycarbonate ５２
• 3.2.5 Characterization of the reactants and products ５３
• 3.3 Results and Discussion ５３
• 3.3.1 Catalyst Screening ５３
• 3.3.2 Parameters related to PC solubility predicted by COSMO-RS ５７
• 3.3.3 Optimization of the reaction parameters for PC ethanolysis ６２
• 3.3.4 Optimization of the PC ethanolysis reaction time using liquid chromatography ６６
• 3.3.5 Improvement in the purity of recovered BPA through DCM washing ６８
• 3.3.6 Reusability of the [EMIM][Ac] ７１
• 3.3.7 Recovery of DEC via distillation-extraction process ７４
• 3.3.8 Repolymerization of PC from BPA obtained via PC ethanolysis ７６
• 3.4 Conclusions ８０
• CHAPTER 4. EFFECT OF HYDROXYL GROUP PRESENCE AND ACCESSIBILITY IN ANIONS ON THE ETHANOLYSIS OF POLYCARBONATE ８１
• 4.1 Introduction ８１
• 4.2 Materials and Methods ８２
• 4.2.1 Materials ８２
• 4.2.2 Halide-to-anion switch for imidazolium-based ionic liquids using AER ８２
• 4.2.3 DFT calculations ８２
• 4.2.4 Protocol for the ethanolysis of PC ８３
• 4.2.5 Characterization of the reactants and products ８４
• 4.3 Results and Discussion ８５
• 4.3.1 Catalyst screening ８５
• 4.3.2 DFT-based structural investigation of hydroxyl group accessibility in carboxylate anions ９０
• 4.3.3 Optimizing reaction parameters of PC ethanolysis using [EMIM][La] ９９
• 4.3.4 Reusability of [EMIM][La] １０１
• 4.3.5 Purification of recovered BPA via DCM Washing １０３
• 4.3.6 Recovery of DEC by distillation and extraction １０５
• 4.4 Conclusions １０７
• CHAPTER 5. CONCLUSION １０９
• REFERENCES １１０